Some other groups have english speaking members like SNSD. Before Kim Ji Wons Descendants of the Sun fame spike she did a soda commercial that had people thinking she wasnt Korean. It is not a requirement to speak Korean.
The reason is that they all generally have that refined Korean celebrity look and because of this we assume that they are.
Mentioned are Idols who have very slight accent to barely any Foreign accent when speaking Korean. Yeri of red velvet who DID speak out on Instagram and Jaehyun and Johnny of NCT who did not. When you hear a Korean celebrity speak fluently in English you might think that he or she is Korean-American or that theyve spent some time overseas. Having Chinese members Japanese members or Thai members makes it easy for fans from other countries to connect with the groups.